1. Field of the Invention
This invention relates to the wet milling of starch bearing materials. The invention will be described with reference to the wet milling of corn, but it will be understood that it can also be applied in other wet milling processes, e.g. those applied to fractions of corn and to wheat, potatoes, etc., for the recovery of starch and/or protein therefrom.
2. The Prior Art
The conventional method of wet milling corn is divided into four steps:
(i) The corn is steeped in water under conditions to soften the grains and the resulting light steep water is separated from the softened grains. Typically, from 0.3 m.sup.3 to more than 1 m.sup.3 of light steep water is produced per ton of corn ground.
(ii) The softened grains are wet milled and the separated by-products, germ, fibres (hull) and gluten are recovered, typically as set masses at an average solids content of 30-50% by weight.
(iii) The starch slurry resulting from wet milling is washed, usually in several stages by means of a counter-current flow of water, to reduce the levels of soluble and insoluble impurities down to desired amounts. For this washing process, from 1.0 m.sup.3 to 2.0 m.sup.3 or more of water is used per ton of corn ground.
(iv) The resulting starch slurry may, optionally, depending on its intended use, be dewatered in which case the water is recycled to the washing step (iii).
Conventionally, water for the process passes counter-current to the starch. Fresh water enters the process at the last washing stage of step (iii) and passes successivley through the washing stages back to the first. Water that has been used in step (iii) passes to the wet milling step (ii). Water from the wet milling step (ii) passes to the steeping step (i) and finally leaves the system as light steep water. The solids content of the water rises on passage through each of these steps, and reaches a level from about 60 to 110 g/l in the light steep water. Most or all of the light steep water is treated, usually by evaporation, to recover the solids which are valuable for various uses, e.g. in animal feedstuffs.
One of the factors which determines the purity of the starch product is the amount of water used in the washing step (iii). For a given purity it is possible to compensate to some extent for a reduction in the amunt of washing water by increasing the number of washing stages. However, even with such refinements, it is not possible to obtain high purity starch without washing it with a lot of water--more water than is required for the earlier steeping and milling steps of the process.
Alternative methods for the recovery of starch and other products from corn or other starch bearing materials have been proposed. For example, U.S. Pat. Nos. 4,171,384 and 4,181,748 to Chwalek and Olson disclose dry-wet milling processes for wheat and corn, respectively, wherein the raw materials are first dry milled and the major portion of fibre and germ are removed after which the remaining portion of the kernel is wet processed. Such systems employ less water than wet milling and therefore make washing of the starch more critical.
Every additional kilogram of fresh water that is introduced into the washing step becomes an additional kilogram of light steep water that has to be evaporated or otherwise treated. It is an object of this invention to provide a process that enables a larger volume of water to be used in the washing step without a concomitant increase in the volume of the light steep water; or conversely, which reduces the amount of light steep water without a concomitant reduction in the amount of water for washing the starch.
Conventional wet processes applied to materials other than whole grains, e.g. potatoes or wheat flour, do not employ steeping steps as such, but all employ final starch washing steps as well as preliminary treatments with water, eg., soaking, slurrying, or conditioning with water.